1. Field of the Invention
The present invention relates generally to the field of direct current-to-direct current (DC/DC) converters, and more particularly to commutation control schemes for an isolated boost converter.
2. Description of the Related Art
Isolated DC/DC converters for converting a low voltage direct current (dc) power source, such as a 12 volt battery, to a high voltage DC power source, such as a 300V traction battery are known in the art. An example of such a converter in the form of an isolated boost DC/DC converter is illustrated in FIG. 1. In such a converter, an inductor Lf is used as the current source at the low voltage side Vb to reduce the RMS (root-mean-square) current rating of low voltage transistors S1, S2, S3, and S4. The low voltage transistors S1, S2, S3, and S4 operate as an inverter to convert DC current (voltage) to a high frequency alternating current (ac) (voltage). An isolation transformer T steps up the voltage to a higher level by the turns ratio, while providing galvanic isolation for safety regulations. Diodes D5, D6, D7, and D8 operate as a rectifier to convert the high frequency AC current (voltage) to the desired high DC voltage.
Referring to FIG. 1, on the input side, the function is basically to chop the low voltage from the Vb energy source, such as the 12 volt battery voltage, into the AC voltage. For this particular arrangement, the inductor Lf is provided to limit the inrush current. Thus, the inductor is provided to regulate the input current to limit the current from the battery. This has a number of advantages, a primary one of which is that smaller devices such as the switches, S1, S2, S3, and S4, with a lower current rating, can be used. Therefore, an advantage of adding the inductor is to limit the current, so that smaller devices can be utilized. A problem in using the inductor is that the converter also includes the transformer T, which posses non-zero leakage inductance Llk. As more fully explained below, this non-zero leakage inductance creates a problem whenever the switch states are changed.
The way in which the primary side of the converter generates a 12 volt plus or minus square wave is that first, switches S1 and S2 are turned on. This connects the A terminal of the transformer to the positive or P battery terminal and the B terminal of the transformer to the negative or N battery terminal. Thus, if the voltage Vab across terminals A and B is plotted as a function of time, the Vab will be plus 12 volts. Thereafter, switches S1 and S2 are turned off, and switches S3 and S4 are turned on instead. Basically, the polarity of Vab is thereby reversed, and the Vab becomes negative 12 volts. Continuing to alternate the switches in this way produces a square waveform, and the DC voltage is changed to AC voltage. Thus, switches S1, S2, S3, and S4 invert the DC voltage to AC voltage and are referred to collectively as the inverter.
Referring once more to FIG. 1, when switches S1 and S2 are on, the current is drawn from the battery Vb and goes through the inductance Lf into the leakage inductance Llk of the transformer and then flows out of terminal B and is returned through switch S2 to the battery Vb. When the polarity is changed by turning off switches S1 and S2 and turning on switches S3 and S4, the current likewise goes through the inductance Lf. However, the current then flows through switch S4 to terminal B first and thereafter through the leakage inductance Llk of the transformer and out from the leakage inductance Llk to terminal A and is then returned through switch S3 to the negative bus N. Thus, it can be seen that the current inside the leakage inductance Llk has a reversed polarity. This process is referred to as commutation.
Whenever there is a change of current in an inductor, there is a voltage across the inductor referred to as Ldl/dt, i.e., the inductance L times the rate of current change dl/dt. The change of the polarity of the current in the leakage inductance occurs as the switches are turned on and off at a level of micro-seconds. Therefore, if the input current is, for example, 150 amperes, it becomes negative 150 amperes in one or two micro-seconds. Thus, the dl/dt is very high, and if the inductance leakage is not at zero, the voltage can be sizeable. For example, the leakage inductance associated with the power stage of a DC/DC converter is typically in the range of ten microhenries. When the current is reversed 300 amperes from plus 150 amperes to negative 150 amperes in half a micro-second, if the leakage inductance is, for example, four or five microhenries, the voltage is in the range of about 2000 volts. The switches S1, S2, S3, and S4, which are power transistors, have a voltage limit, and it can be very difficult for these transistors to withstand such a high voltage spike.
This huge voltage spike can damage the power transistor S1, S2, S3, and S4. A typical approach to dealing with this problem is a passive clamp circuit, as shown in FIG. 1, which is basically a capacitor with a diode. When the voltage is beyond the capacitor voltage, the diode conducts and diverts the energy to the capacitor to clamp out the voltage spike. In that way, the switches S1, S2, S3, and S4 can be protected from an over-voltage situation. However, each time the voltage is clamped, energy that is transferred from the leakage inductance Llk is stored in the clamp capacitor. That energy will charge up the capacitor unless a way is provided to deplete the capacitor or to consume the energy each time the transistors are switched. That is usually done by a resistor in parallel with the capacitor to bleed out the capacitor voltage. Each time the polarity of the current in the leakage inductance Llk is switched, the energy stored in the leakage inductor is lost and therefore wasted if a passive clamp circuit is used.